Dual-circuit chiller with two-pass heat exchanger in a series counterflow arrangement

ABSTRACT

A dual refrigeration circuit, watercooled chiller has its respective evaporators and condensers interconnected by waterboxes, with each waterbox having an inlet flow and outlet flow connection, and with three passages interconnected with the respective evaporators/condensers of the first and second circuit, and with each of the condensers/evaporators having return bends at their ends to provide a two-pass flow arrangement. The flow in the condenser waterbox passes into a first passage and then in one direction to the condenser of one circuit while the flow into the evaporator waterbox passes into a first passage and then in the opposite direction to one of the circuit evaporators. In this manner, a series counterflow arrangement with two water passes is achieved.

BACKGROUND OF THE INVENTION

This invention relates generally to water cooled chillers and, morespecifically, to the interconnection of two vapor compressionrefrigeration systems in a series-counterflow arrangement.

Water cooled chillers in a series-counterflow arrangement consist of twoindependent vapor compression refrigeration systems with chilled waterand condenser water circuits that are common to both circuits and arearranged in series. This arrangement allows for an increased coefficientof performance (COP) over a single refrigeration circuit design becausethe separate circuits with series counterflow have a lower averagepressure differential between the evaporator and condenser, thusrequiring less energy to compress refrigerant from the evaporator to thecondenser.

In such a system, water in each of the evaporators and the condensersflows through a plurality of tubes that span both refrigerationcircuits, with the refrigeration circuits being separated by a tubesheetwhich is located at the middle of the tubes, and with each tube beinghermetically sealed to the tubesheet, typically by expansion of the tubeto the tubesheet.

One problem that arises is that of servicing the tubes such as may berequired if a tube fails in operation. Such removal of a tube requirescutting the tube at all locations where it has been expanded and thenpulling the tube out. It is not possible to completely remove a tubesince there is no access to cut the tube at the center tubesheetlocation, which is inside the refrigerant boundary. If a tube is cutinternally, or if a tube fails in operation, a leak path is createdbetween the circuits that does not allow for operation of eithercircuit, thus adversely impacting both reliability and serviceability.

Another problem with a dual circuit system is that of control. Acritical parameter for control of a water cooled chiller is the use ofthe leaving temperature differential, which is the difference in thetemperature of the water leaving a heat exchanger and the refrigeranttemperature within the heat exchanger. Since the water tubes span bothrefrigerant circuits in a dual system, it is not possible to obtain theleaving water temperatures of the upstream circuit's condenser orevaporator.

In addition to serviceability and control as discussed hereinabove,prior art heat exchanger tubes that span dual circuits pose problems ofreliability, accessibility, shipping and performance. That is, becausethe common tubes extend across both circuits, it is impossible tooptimize the heat transfer tubes in each circuit independently, andshipping of machines that are longer due to the longer tubes can bedifficult.

It is desirable to have a two water pass arrangement, wherein enteringand leaving water connections can be made from the same location on thechiller, thus allowing access to a tubesheet of the cooler and condenseron the non-connection end without requiring removal of the water pipingto the chiller for cleaning or replacing tubes. Also, for those skilledin the art, a two pass arrangement can be desirable for obtaining higherwater velocities in the heat exchanger tubes while maintaining a fixednumber of heat exchanger tubes. This invention allows for two pass heatexchangers with a series counterflow arrangement by way of a novelmachine arrangement and waterbox design.

SUMMARY OF THE INVENTION

Briefly, in accordance with one aspect of the invention, each circuithas unique tubesheets that separate the refrigeration circuit from thecooling medium. Between each circuit is an intermediate waterbox thatpasses water from the upstream circuit to the downstream circuit. Thewaterbox is removable for service and enables the transporting of theunits in pieces with shorter length requirements.

In accordance with another aspect of the invention, since each circuithas its separate and unique tubes, a tube failure in either circuit nolonger creates a refrigerant leak path to the adjacent circuit, suchthat operation of the nonfailed circuit can be maintained, therebyincreasing reliability.

By another aspect of the invention, since the intermediate waterbox isaccessible from the outside, temperature measurement instrumentation canbe installed to obtain the leaving temperature differential of theupstream circuit, thereby providing better control of the system.

In accordance with another aspect of the invention, provision is made inboth the cooler and condenser for the entering and leaving waterconnections to be made at the same location on the intermediatewaterbox, thus greatly facilitating access thereto.

By another aspect of the invention, each of the cooler and condenserintermediate waterboxes have three separate passages, and the enteringand leaving water directions are reversed in the respective cooler andcondenser waterboxes such that the respective flows are in a seriescounterflow arrangement.

In the drawings as hereinafter described, a preferred embodiment isdepicted; however, various other modifications and alternateconstructions can be made thereto without departing from the spirit andscope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of the temperatures in a singlecircuit chiller in accordance with the prior art.

FIG. 2 is a schematic illustration of the temperatures in a dual-circuitchiller in accordance with the prior art.

FIG. 3 is a schematic illustration of the condensers and evaporators ofa dual-circuit chiller in accordance with the prior art.

FIG. 4 is a schematic illustration of dual-circuit chiller system inaccordance with one aspect of the present invention.

FIG. 5 is a schematic illustration of the condenser and evaporators in adual-circuit system of one aspect of the present invention.

FIG. 6 is a schematic illustration of the waterbox portion of thedual-circuit system in accordance with one aspect of the presentinvention.

FIG. 7 is a perspective view of the waterbox portions of a dual-circuitsystem in accordance with one aspect of the present invention.

FIG. 8 is an end view of the waterbox portion of a dual-circuit systemin accordance with one aspect of the present invention.

FIG. 9 is a schematic illustration of a waterbox arrangement inaccordance with another aspect of the present invention.

FIG. 10 is a further illustration thereof to show the flow directionsand relationships thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a condenser 11 and a cooler or evaporator 12 of a singlecircuit chiller that is typical of the prior art. As shown, thecondenser water and evaporator water flows in a counterflowrelationship, and the resulting temperatures entering and leaving thecondenser and evaporator are as shown.

In order to obtain increased COPs, a dual-circuit is connected in seriescounterflow arrangement as shown in FIG. 2. Here, two independent vaporcompression refrigeration circuits, 13 and 14, are connected by anintermediate tubesheet 15 as shown. The first circuit 13 has a condenser16 and an evaporator 17, and the second circuit 14 has its own condenser18 and evaporator 19. However, the condenser water circuits of thecondenser 16 and 18 are common to both circuits and are arranged inseries. Also, the chilled water circuits of the evaporators 17 and 19are common to both circuits and are arranged in series. This can be bestseen by reference to FIG. 3.

It will be seen in FIG. 3 that the condenser tubes 21 are long and spanthe length of each of the condensers 16 and 18 of the circuits 13 and14. While the intermediate tubesheet 15 isolates and separates therefrigerant in the respective circuits 13 and 14, the water flow throughthe condenser tubes 21 is continuous from the entrance of the condenser16 to the outlet of the condenser 18.

Similarly, the evaporator tubes 22 are unitary members that extendacross both circuits 13 and 14, with the intermediate tubesheetsproviding isolation only for the refrigerant in the systems 13 and 14,but allow for the evaporator water to flow continuously from the inletend of the evaporator 19 to the outlet end of the evaporator 17.

The series counterflow effect is achieved by separation of the heatexchangers into two isolated circuits. With typical refrigerant heatexchangers, the saturation conditions for the cooler and condenser are afunction of the leaving water temperature from each circuit. With asingle circuit chiller, shown in FIG. 2, typical leaving watertemperatures for the cooler and condenser would be 44 F and 95 F,respectively. An efficient water/refrigerant heat exchanger would have adifference in temperature between the leaving water and the refrigerant,or LTD, of approximately 1 degree F., thus in the single circuit case,the saturation temperatures would be 43 F in the cooler, and 96 F in thecondenser, see FIG. 1. The resulting lift is the difference, or 53degrees F. In a two circuit design with equivalent refrigeration effectin each circuit, the water temperature in the middle of the two circuitsis approximately the mean of the entering and leaving temperatures. Inthe example of FIG. 2 above, the temperature in between the cooler andcondenser circuits would be 49 F and 90 F, respectively. With typicalheat exchanger LTD's, the saturation conditions for the two coolercircuits would then be approximately 48 F and 43 F, and the saturationconditions for the two condensers would be approximately 96 F and 91 F.With the series counterflow design, the cooler and condenser water enterfrom opposite ends, therefore the cooler and condenser circuits arepaired so that the higher saturation cooler is on the same circuit withthe higher saturation temperature condenser, and the two lowersaturations heat exchangers are paired. The result is that eachrefrigerant circuit has the same lift, and the lift for each circuit isless than the single circuit design. In the examples described above,the single circuit lift was 53 degrees F. and the series counterflowlift was 48 degrees F. The series counterflow arrangement hasapproximately 10% less lift, thus greater system efficiency.

As discussed hereinabove, such dual-circuit systems with heat exchangertubes that span both circuits present problems with respect to service,reliability, shipping, performance, control and accessibility.

Referring now to FIG. 4, a system is shown to overcome theabove-discussed problems. A first circuit, 23, includes a condenser 24,an expansion device 26, an evaporator 27 and a compressor 28, whichoperate in serial flow relationship in a well-known manner. A secondcircuit, 29, includes a condenser 31, an expansion device 32, anevaporator 33 and a compressor 34 which also are connected in serialflow relationship and operate in a well known manner. The two circuits23 and 29 are interconnected in a manner similar to that shown in FIG. 3but with a different structure at the interface between the two circuitsand different structure with respect to the tubes within both thecondensers and the evaporators.

As shown in FIGS. 4 and 5, at an intermediate position between the twoevaporators 27 and 33 is an evaporator waterbox 36, and at anintermediate position between the two condensers 24 and 31 is acondenser waterbox 37. Further, unlike the systems described hereinabovewherein the tubes are unitary tubes extending across both circuits, thecondenser tubes 38 of circuit 1 are separate and independent from thecondenser tubes 39 of circuit 2, and the evaporator tubes 41 in circuit1 are separate and distinct from the evaporator tubes 42 of circuit 2.That is, the condenser tubes 38 are fluidly connected to one side of thewaterbox 36 and the condenser tubes 39 are fluidly connected to theother side thereof. Similarly, the evaporator tubes 41 are fluidlyconnected to one side of the waterbox 37 and the evaporator tubes 42 arefluidly connected to the other side thereof The waterboxes 36 and 37therefore act as intermediate receptacles for the water as it passesbetween the first circuit 23 and second circuit 29.

The advantages of the above-described design are numerous. First of all,rather than having long unitary tubes, the tubes, and therefore therefrigeration circuits, are generally only about half as long and can bemore easily handled and shipped to a site, with the tubes, and thereforethe refrigeration circuits, being independent and separatable from thewaterboxes. Second, since the tubes are independent, they can beconfigurable to optimize performance in each circuit. That is, inaddition to the variation in length of the tubes in each circuit, thenumber of tubes within the second circuit can be different from those inthe first circuit as shown in FIG. 5, and other variations can be made,such as different tube material, or different heat transferenhancements. This allows the designer to optimize the desired capacity,efficiency, pressure drop, or cost for each circuit.

Other advantages of the present system can be seen by reference to FIG.6. Because the water from the upstream tubes is discharged along oneside of the waterbox 36 (or waterbox 37 in the case of the evaporator),it tends to cause a turbulence within the waterbox such that theindividual flow streams are mixed so as to become a reservoir of waterwith a relative uniform temperature before it enters the tubes of thedownstream circuit. This mixing is beneficial to the heat transfereffectiveness, thereby increasing COP of the total system.

By using the waterbox 36 as described, the intermediate waterbox 36 isnow accessible from the outside and temperature measurementinstrumentation 43 can easily be used to obtain the leaving temperaturedifferential of the upstream heat exchangers, thus providing improvedcontrol of the system.

Another advantage of the use of waterboxes as described is that offacilitating service and repair. That is, since the waterbox is attachedto the tube circuits in a manner that allows removal of the waterbox, aswill be described hereinafter, the removal of the waterbox allowsservice of the tubes at each circuit's tubesheet, thereby substantiallyimproving serviceability. Further, since a tube failure in eithercircuit does not create a refrigerant leak path to the adjacent circuit,the reliability of the system is substantially enhanced.

Referring now to FIGS. 7 and 8, the structural interface of theintermediate waterbox and the adjacent circuits are shown. As shown theintermediate waterbox 44 comprises a relatively short cylinder with aplurality of holes 46 formed longitudinally from one end 47 to theother, for receiving bolts 48 passing through the respective tubesheets49 and 51. The waterbox, 44, is thus sandwiched between the tubesheets49 and 51 of the respective circuits and can be easily disassembled byremoving the bolts, 48, to get access to the tubes for repair purposesat the tubesheets between the circuits. It will therefore be recognizedthat each of the circuits is independent, and access can be gained tothe intermediate tube to tubesheet joints without disrupting refrigerantboundary of either circuit.

Although the waterbox 44 is shown in FIGS. 7 and 8 as relatively shortin length (i.e. about 4 inches), its configuration, size and shape canbe substantially varied while remaining within the scope of the presentinvention. Further, although described in terms of use with a watercooled chiller, the present invention could also be applicable to an aircooled chiller wherein the evaporators of series connected circuits areinterconnected by way of an intermediate waterbox structure.

The embodiments of the invention as described hereinabove relate only toa single pass heat exchanger relationship. In order to obtain a two-passarrangement, the intermediate waterboxes and the various leaving andentering connections must be significantly modified as are shown inFIGS. 9 and 10 and as will now be described.

Rather than having tubes that make a single pass through the heatexchangers, each of the circuits #1 and #2, 52 and 53, respectively,have their heat exchangers arranged such that the fluid makes two passesthrough each of the heat exchangers. That is, rather than the waterentering at one end of the cooler and condenser as describedhereinabove, the water enters and leaves the intermediate waterboxes 54and 56, respectively, and then passes through each of the heatexchangers twice before leaving the respective waterboxes. In order forthis to occur, each of the heat exchangers must have their tubesinterconnected at their ends by way of return bends. Thus, within thecondensers 57 of the circuits #1 and #2, the heat exchanger 58 hasreturn bend 59, and the heat exchanger 61 has return bend 62. Similarly,in the cooler 63, heat exchanger 64 has return bend 66 and heatexchanger 67 has return bend 68.

The manner in which the water enters and leaves the circuits will now bedescribed with reference to FIG. 10. The intermediate waterbox 56 forthe cooler circuits 63 is divided into three passages 69, 71 and 72 asshown. The entering water flows into passage 69, then flows to the heatexchanger 67 where it passes first through pass 1, a return bend 68 andthen pass 2 before it enters the passage 71 in the waterbox 56. It thenpasses into the heat exchanger 64, first through pass 1, then throughthe return bend 66 and then pass 2, before it enters the passage 72 ofthe waterbox 56 and then leaves the cooler.

In the condenser 57, the water flows into the intermediate waterbox 54and then flows in the opposite direction from the water flowing from thewaterbox 56 to the heat exchanger 67 (i.e. to the heat exchanger 58)where it passes first through a first pass, then through the return bend59 and then back through the second pass, after which it passes into themiddle passage of the waterbox 54. Note that the direction of flow is inthe opposite direction from the flow in the middle passage 71 of thewaterbox 56. It then passes into the heat exchanger 61, flowing firstthrough a first pass, then through the return bend 62 and then throughthe second pass, prior to entering the waterbox 54 from which it thenleaves.

It will thus be seen that, by the use of the intermediate waterboxes 54and 56, and the selective direction of flow in each of the condensers 57and the cooler 63, a two-pass, series counterflow arrangement isobtained. Further, the interconnections for the entering and leavingwater in each of the intermediate waterboxes 54 and 56 are commonlylocated at the waterboxes themselves, thus facilitating easy accessthereto.

1. A chiller system of the type having first and second refrigerationcircuits with each refrigeration circuit having a compressor, acondenser, an expansion device and an evaporator and with the respectiveevaporators in the first and second circuits having a plurality of tubesto conduct the flow of fluid to be cooled, and with the respectiveevaporators of the first and second circuits being interconnected inseries relationship such that the fluid to be chilled passes seriallythrough the respective evaporators of the first and second circuits,comprising: an evaporator waterbox interconnected between theevaporators of the first and second circuits and having at least threepassages therein with the first passage having a water inlet connectionand a second passage having a water outlet connection; each of theevaporators of the first and second circuits having first and secondpass tubes interconnected at their ends by a return bend; such that thewater flows into said first passage and then into one of the evaporatorsof the first and second circuit, flowing serially through said firstpass, said return bend and through said second pass, and then into athird passage of said evaporator waterbox prior to flowing through saidother evaporator, flowing serially through said first pass, said returnbend and through said second pass and then into said second passage andout the water outlet connection.
 2. A chiller system as set forth inclaim 1 wherein said respective condensers in the first and secondcircuits have a plurality of tubes to conduct the flow of fluid to becooled, and with the respective condensers of the first and secondcircuits being interconnected in series relationship such that the fluidto chilled passes serially through the respective evaporators of thefirst and second circuits; a condenser waterbox interconnected betweenthe condensers of the first and second circuits and having at leastthree passages therein with the first passage having a water inletconnection and a second passage having a water outlet connection; eachof the condensers of the first and second circuits having first andsecond pass tubes interconnected at their ends by a return bend; suchthat the water flows into said first passage and then into one of thecondensers of the first or second circuit, flowing serially through saidfirst pass, said return bend and through said second pass and then intoa third passage of said condenser waterbox prior to flowing to saidother condenser, flowing serially through said first pass, said returnbend and through said second pass and then into said second passage ofsaid condenser waterbox to said water outlet connection.
 3. A chillersystem as set forth in claim 2 wherein said condenser and evaporatorwaterboxes are so connected that the respective flows in the thirdpassages of the respective condenser and evaporator are in oppositedirections.
 4. A chiller system as set forth in claim 2 wherein thedirection of the water flowing from said evaporator waterbox to one ofsaid evaporators is in an opposite direction from the flow of waterflowing from said condenser waterbox to one of said condensers.
 5. Adual-circuit chiller, comprising: a first circuit having a compressor, acondenser, an expansion device and an evaporator, with the evaporatorhaving at least one tube for conducting the flow of water to be cooledfrom an inlet end to a return bend and back to an outlet end of thetube; a second circuit having a compressor, a condenser, an expansiondevice and an evaporator with the evaporator having at least one tubefor conducting the flow of water to be cooled from an inlet end to areturn bend and back to an outlet end of the tube; and an evaporatorwaterbox having inlet and outlet flow openings and being fluidlyinterconnected between said first circuit tube inlet and outlet ends andthe second circuit tube inlet and outlet ends, such that water to becooled flows into said evaporator waterbox, through said first circuittube, back into said evaporator waterbox, through said second circuittube, back into said evaporator waterbox and then out said outlet flowopening.
 6. A dual-circuit chiller as set forth in claim 5 andincluding: each of said first and second circuit condensers having atleast one tube for conducting the flow of water to be cooled from aninlet to a return bend and back to an outlet end of the tube; acondenser waterbox having inlet and outlet flow openings and beingfluidly interconnected between said first tube inlet and outlet ends andthe second circuit tube inlet and outlets ends, such that water be tocooled flows into said condenser waterbox, through said second circuittube, back into said condenser waterbox, through said first circuit tubeand back into said condenser waterbox, and then out said outlet flowopening.
 7. A dual-circuit chiller as set forth in claim 6 wherein eachof said evaporator and condenser waterboxes have three passages formedtherein, with a first passage having the inlet flow opening a secondpassage having the outlet flow opening and a third passage for fluidlyconnecting the two evaporators/condenser.
 8. A chiller system as setforth in claim 7 wherein said condenser and evaporator waterboxes are soconnected that the respective flows in the third passages of therespective condenser and evaporator are in opposite directions.
 9. Achiller system as set forth in claim 7 wherein direction of the waterflowing from said evaporator waterbox to said first circuit tube is inan opposite direction from the flow of water flowing from said condenserwaterbox to said second circuit tube.